1. Field of the Invention
This invention relates to a method for producing a useful stream of hot gas from a fluidized bed combustor while controlling the bed's temperature. The invention further relates to an improved fluidized bed combuster.
2. Description of the Prior Art
Fluidized bed combustors are useful for providing heat for useful purposes. In conventional fluidized bed combustors, a fluidized bed is created by blowing air up through an initially static quantity of particulate matter, such as crushed stone, which partially fills a combustion chamber. By bringing the air flow up to and controlling it within a certain range of velocities, the particles will rise and be sustained in a turbulent suspension, thereby forming a fluidized bed.
Such fluidized beds can provide an ideal environment for the combustion of many materials, including solid fuels such as coal. By preheating a fluidized bed and introducing a fuel (e.g., coal) into the bed at a controlled rate, a self-sustaining combustion reaction can be made to occur. The heat released by such combustion is transferred to the bed material and the air and combustion produce gases which pass through the bed.
Fluidized bed combustors are useful for controlling the emission of undesirable combustion by-products. For example, by injecting a sorbent material such as limestone into the fluidized bed combustor, sulfur dioxide emissions which always accompany the burning of coal can be substantially reduced. This injected sorbent material and its subsequent reaction product (i.e., CaSO.sub.4) are solids and become part of the bed material. As bed material is drawn from the bed to control bed height, or as it is elutriated and eventually captured in, for example, a baghouse, the sulfur retained is removed from the system. This dry solid material is often more easily disposed of than the wet sludges resulting from competing downstream desulfurization processes.
The bed temperature of a fluidized bed combustor is generally limited on the upper side by several factors. Most solid fuels of interest have some ash content. Such ashes become sticky and therefore tend to agglomerate at some temperature level (typically somewhere in 1900.degree.-2400.degree. F. range). Even more restrictive, generally, is the temperature requirement for an effective sulfur dioxide reaction with the sorbent. At atmospheric pressure this reaction becomes rapidly less effective as bed temperatures exceed 1650.degree. F.
Almost all fuels of commercial value, including coal, have adiabatic flame temperatures, for stoichiometric proportions of fuel and air, which exceed 1650.degree. F. Most of these adiabatic flame temperatures also exceed the ash fusion value. Consequently, the temperature of a fluidized bed must generally be kept below the stoichiometric adiabatic value by extracting heat from the bed.
Methods for extracting heat from a bed in order to lower its temperature to acceptable levels can also aid the combustor's primary purpose of providing heat for useful purposes. Several known methods for extracting heat from a fluidized bed involve transferring heat from the bed material to a medium. Such transfer of heat from the bed can be utilized to control the bed's temperature. Such transfer of heat to the medium can also be utilized to provide heat for useful purposes by conveying the medium containing such heat to the point of use. The following two are the most common of such methods.
One common method uses heat exchanger surfaces immersed in or bounding the bed. The medium to be heated (i.e. a liquid such as water, a vapor such as steam, or a gas) is passed through the heat exchanger where it receives heat from the bed by conduction through the heat exchanger surfaces. The medium then conveys the heat to the use points. However, although this method is useful, and for some situations even the preferred one, it often adds considerably to the system cost since at even relatively modest temperatures the often corrosive nature of the fluidized bed combustor environment may dictate the use of expensive materials and frequent maintenance.
The second common method for extracting heat from a fluidized bed is simply to pass air through the bed in quantities exceeding the optimal amount required for efficient combustion of the fuel. This excess air along with the combustion products are now the heat conveying medium. The shortcomings of this approach include high power consumption since this extra air must be compressed appreciably to force it through the fluidized bed and the bed's air distribution grid. Another shortcoming is that passing excess air through the bed requires that the bed's cross sectional area be increased relative to the cross sectional area required for efficient combustion, so as not to create excessive gas velocities in the bed. Excessive velocities are those at which any of the following problems occur:
1. Bed blows away rapidly. PA1 2. Combustion efficiency is poor. PA1 3. Sulfur removal by sorbent reaction is poor. PA1 4. Bed depth required for good combustion efficiency or sulfur retention has become excessive. PA1 1. Production of steam in a waste heat boiler (being either a new boiler or an old boiler previously fired by some premium fuel such as oil or natural gas); PA1 2. Drying of lumber (by using the stream of gas in lumber dry kiln); and PA1 3. Drying of various minerals (examples are bauxite and coal).
A larger bed area implies a more costly combustor since it requires a larger more costly air distribution grid with which to introduce air into the bed. Also, the vessel or box which contains the combustor increases in size and cost.
In summary, the commonly used methods for extracting heat from fluidized bed combustors involve significant capital and operating costs.